Arrays of light emitting diodes (“LEDs”) are utilized for a wide variety of applications, including for general lighting and multicolored lighting. Because emitted light intensity is proportional to the average current through an LED (or through a plurality of LEDs connected in series), adjusting the average current through the LED(s) is one typical method of regulating the intensity or the color of the illumination source.
Because a light-emitting diode is a semiconductor device that emits incoherent, narrow-spectrum light when electrically biased in the forward direction of its (p-n) junction, the most common methods of changing the output intensity of an LED biases its p-n junction by varying either the forward current (“I”) or forward bias voltage (“V”), according to the selected LED specifications, which may be a function of the selected LED fabrication technology. For driving an illumination system (e.g., an array of LEDs), electronic circuits typically employ a converter to transform an AC input voltage (e.g., AC line voltage, also referred to as “AC mains”) and provide a DC voltage source, with a linear “regulator” then used to regulate the lighting source current. Such converters and regulators are often implemented as a single unit, and may be referred to equivalently as either a converter or a regulator.
Pulse width modulation (“PWM”), in which a pulse is generated with a constant amplitude but having a duty cycle which may be variable, is a common prior art technique for regulating average current and thereby adjusting the emitted light intensity (also referred to as “dimming”) of LEDs, other solid-state lighting, LCDs, and fluorescent lighting, for example. See, e.g., Application Note AN65 “A fourth generation of LCD backlighting technology” by Jim Williams, Linear Technology, November 1995 (LCDs); Vitello U.S. Pat. No. 5,719,474 (dimming of fluorescent lamps by modulating the pulse width of current pulses); and Ihor Lys et al., U.S. Pat. Nos. 6,340,868 and 6,211,626, entitled “Illumination components” (pulse width modulated current control or other form of current control for intensity and color control of LEDs). In these applications for LEDs, a processor is typically used for controlling the amount of electrical current supplied to each LED, such that a particular amount of current supplied to the LED module generates a corresponding color within the electromagnetic spectrum.
Such current control for dimming may be based on a variety of modulation techniques, such as PWM current control, analog current control, digital current control and any other current control method or system for controlling the current. For example, in Mueler et al., U.S. Pat. Nos. 6,016,038; 6,150,774; 6,788,011; 6,806,659, and 7,161,311, entitled “Multicolored LED Lighting Method and Apparatus”, under the control of a processor (or other controller), the brightness and/or color of the generated light from LEDs is altered using pulse-width modulated signals, at high or low voltage levels, with a preprogrammed maximum current allowed through the LEDs, in which an activation signal is used for a period of time corresponding to the duty cycle of a PWM signal (with the timing signal effectively being the PWM period). See also U.S. Pat. Nos. 6,528,934; 6,636,003; 6,801,003; 6,975,079; 7,135,824; 7,014,336; 7,038,398; 7,038,399 (a processor may control the intensity or the color by providing a regulated current using a pulse modulated signal, pulse width modulated signals, pulse amplitude modulated signals, analog control signals and other control signals to vary the output of LEDs, so that particular amount of current supplied generates light of a corresponding color and intensity in response to a duty cycle of PWM); and U.S. Pat. No. 6,963,175 (pulse amplitude modulated (PAM) control).
These prior art methods of controlling time averaged forward current of LEDs using different types of pulse modulations, at constant or variable frequency, by switching the LED current alternatively from a predetermined maximum value toward a lower value (including zero), creates electromagnetic interference (“EMI”) problems and also suffers from a limitation on the depth of intensity variation. Analog control/Constant Current Reduction (or Regulation) (“CCR”), which typically varies the amplitude of the supplied current, also has various problems, including inaccurate control of intensity, especially at low current levels (at which component tolerances are most sensitive), and including instability of LED performance at low energy biasing of the p-n junction, leading to substantial wavelength shifting and corresponding color distortions.
As described in greater detail below with reference to FIGS. 1-3, both the PWM and CCR techniques of adjusting brightness also result in shifting the wavelength of the light emitted, further resulting in color distortions which may be unacceptable for many applications. The various prior art methods of addressing such color distortions, which are perceptible to the human eye and which can interfere with desired lighting applications, have not been particularly successful. For example, in McKinney et al. U.S. Pat. No. 7,088,059 analog control is used over a first range of intensities, while PWM or pulse frequency modulation (“PFM”) control and analog control is used over a second range of illumination intensities. In Mick U.S. Pat. No. 6,987,787, PWM control is used in addition to variable current control, to provide a much wider range of brightness control by performing a “multiplying” function to the two control inputs (peak current control and PWM control). Despite some improvement of intensity control and color mixing of these two patents, however, the proposed combinations of averaging techniques still do not address the resulting wavelength shifting and corresponding perceived color changes when these techniques are executed, either as a single analog control or as a combination of pulse and analog controls.
Depending on a required quality of the light source, this wavelength change may be tolerated, assuming the reduced quality of the light is acceptable. It has been proposed to correct this distortion through substantially increasing the complexity and cost of the control system by adding emission (color) sensors and other devices to attempt to compensate for the emission shift during intensity regulation. See Application Brief AB 27 “For LCD backlighting Luxeon DCC” Lumiledes, January 2005, at FIG. 5.1 (Functional model of Luxeon DCC driver).
Accordingly, a need remains for an apparatus, system and method for controlling the intensity (brightness) of light emissions for solid state devices such as LEDs, while simultaneously providing for substantial stability of perceived color emission and control over wavelength shifting, over both a range of intensities and also over a range of LED junction temperatures. Such an apparatus, system and method should be capable of being implemented with few components, and without requiring extensive feedback systems.